Download Chapter 8 Operational Amplifier as A Black Box 8.1

Fundamentals of Microelectronics
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CH1
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CH8
Why Microelectronics?
Basic Physics of Semiconductors
Diode Circuits
Physics of Bipolar Transistors
Bipolar Amplifiers
Physics of MOS Transistors
CMOS Amplifiers
Operational Amplifier As A Black Box
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Chapter 8
Operational Amplifier as A Black
Box
 8.1 General Considerations
 8.2 Op-Amp-Based Circuits
 8.3 Nonlinear Functions
 8.4 Op-Amp Nonidealities
 8.5 Design Examples
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Chapter Outline
CH8 Operational Amplifier as A Black Box
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Basic Op Amp
Vout  A0 Vin1  Vin 2 
 Op amp is a circuit that has two inputs and one output.
 It amplifies the difference between the two inputs.
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Inverting and Non-inverting Op Amp
 If the negative input is grounded, the gain is positive.
 If the positive input is grounded, the gain is negative.
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Ideal Op Amp
 Infinite gain
 Infinite input impedance
 Zero output impedance
 Infinite speed
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Virtual Short
Vin1
Vin2
 Due to infinite gain of op amp, the circuit forces Vin2 to be
close to Vin1, thus creating a virtual short.
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Unity Gain Amplifier
Vout  A0 (Vin  Vout )
Vout
A0

Vin 1  A0
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Op Amp with Supply Rails
 To explicitly show the supply voltages, VCC and VEE are
shown.
 In some cases, VEE is zero.
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Noninverting Amplifier (Infinite A0)
Vout
R1
 1
Vin
R2
 A noninverting amplifier returns a fraction of output signal
thru a resistor divider to the negative input.
 With a high Ao, Vout/Vin depends only on ratio of resistors,
which is very precise.
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Noninverting Amplifier (Finite A0)
Vout 
R1   
R1  1 
 1 
 1  1 


Vin
R
R
A


2 
2 
0
 The error term indicates the larger the closed-loop gain, the
less accurate the circuit becomes.
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Extreme Cases of R2 (Infinite A0)
 If R2 is zero, the loop is open and Vout /Vin is equal to the
intrinsic gain of the op amp.
 If R2 is infinite, the circuit becomes a unity-gain amplifier
and Vout /Vin becomes equal to one.
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Inverting Amplifier
0  Vout Vin

R1
R2
Vout  R1

Vin
R2
 Infinite A0 forces the negative input to be a virtual ground.
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Another View of Inverting Amplifier
Inverting
CH8 Operational Amplifier as A Black Box
Noninverting
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Gain Error Due to Finite A0
Vout
R1  1  R1 
  1  1  
Vin
R2  A0  R2 
 The larger the closed loop gain, the more inaccurate the
circuit is.
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Complex Impedances Around the Op Amp
Vout
Z1

Vin
Z2
 The closed-loop gain is still equal to the ratio of two
impedances.
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Integrator
Vout
1

Vin
R1C1s
CH8 Operational Amplifier as A Black Box
Vout

1

Vin dt
R1C1
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Integrator with Pulse Input

1
V1
Vout  
Vin dt  
t 0  t  Tb
R1C1
R1C1
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Comparison of Integrator and RC Lowpass Filter
 The RC low-pass filter is actually a “passive” approximation
to an integrator.
 With the RC time constant large enough, the RC filter
output approaches a ramp.
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Lossy Integrator
Vout
1

Vin 1  1 
 1   R1C1s
A0  A0 
 When finite op amp gain is considered, the integrator
becomes lossy as the pole moves from the origin to 1/[(1+A0)R1C1].
 It can be approximated as an RC circuit with C boosted by a
factor of A0+1.
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Differentiator
Vout
dVin
  R1C1
dt
CH8 Operational Amplifier as A Black Box
Vout
R1

  R1C1s
1
Vin
C1s
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Differentiator with Pulse Input
Vout   R1C1V1 (t )
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Comparison of Differentiator and High-Pass Filter
 The RC high-pass filter is actually a passive approximation
to the differentiator.
 When the RC time constant is small enough, the RC filter
approximates a differentiator.
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Lossy Differentiator
Vout
 R1C1s

Vin 1  1  R1C1s
A0
A0
 When finite op amp gain is considered, the differentiator
becomes lossy as the zero moves from the origin to –
(A0+1)/R1C1.
 It can be approximated as an RC circuit with R reduced by a
factor of (A0+1).
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Op Amp with General Impedances
Vout
Z1
 1
Vin
Z2
 This circuit cannot operate as ideal integrator or
differentiator.
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Voltage Adder
Vout
Ao
Vout
 V1 V2 
  RF   
 R1 R2 
 RF

V1  V2 
R
If R1 = R2=R
 If Ao is infinite, X is pinned at ground, currents proportional
to V1 and V2 will flow to X and then across RF to produce an
output proportional to the sum of two voltages.
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Precision Rectifier
 When Vin is positive, the circuit in b) behaves like that in a),
so the output follows input.
 When Vin is negative, the diode opens, and the output drops
to zero. Thus performing rectification.
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Inverting Precision Rectifier
 When Vin is positive, the diode is on, Vy is pinned around
VD,on, and Vx at virtual ground.
 When Vin is negative, the diode is off, Vy goes extremely
negative, and Vx becomes equal to Vin.
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Logarithmic Amplifier
Vout
Vin
 VT ln
R1 I S
 By inserting a bipolar transistor in the loop, an amplifier
with logarithmic characteristic can be constructed.
 This is because the current to voltage conversion of a
bipolar transistor is a natural logarithm.
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Square-Root Amplifier
Vout  
2Vin
 VTH
W
 n Cox R1
L
 By replacing the bipolar transistor with a MOSFET, an
amplifier with a square-root characteristic can be built.
 This is because the current to voltage conversion of a
MOSFET is square-root.
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Op Amp Nonidealities: DC Offsets
 Offsets in an op amp that arise from input stage mismatch
cause the input-output characteristic to shift in either the
positive or negative direction (the plot displays positive
direction).
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Effects of DC Offsets
Vout
 R1 
 1  Vin  Vos 
 R2 
 As it can be seen, the op amp amplifies the input as well as
the offset, thus creating errors.
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Saturation Due to DC Offsets
 Since the offset will be amplified just like the input signal,
output of the first stage may drive the second stage into
saturation.
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Offset in Integrator
Vout
R2
1

Vin
R1 R2C1s  1
 A resistor can be placed in parallel with the capacitor to
“absorb” the offset. However, this means the closed-loop
transfer function no longer has a pole at origin.
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Input Bias Current
 The effect of bipolar base currents can be modeled as
current sources tied from the input to ground.
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Effects of Input Bias Current on Noninverting
Amplifier
 R1 
Vout   R2 I B 2     R1 I B 2
 R2 
 It turns out that IB1 has no effect on the output and IB2
affects the output by producing a voltage drop across R1.
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Input Bias Current Cancellation
 R1 
Vout  Vcorr 1    I B 2 R1
 R2 
 We can cancel the effect of input bias current by inserting a
correction voltage in series with the positive terminal.
 In order to produce a zero output, Vcorr=-IB2(R1||R2).
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Correction for  Variation
I B1  I B 2
 Since the correction voltage is dependent upon , and 
varies with process, we insert a parallel resistor
combination in series with the positive input. As long as
IB1= IB2, the correction voltage can track the  variation.
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Effects of Input Bias Currents on Integrator
Vout
1

R1C1

 I B 2 R1 dt
 Input bias current will be integrated by the integrator and
eventually saturate the amplifier.
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Integrator’s Input Bias Current Cancellation
 By placing a resistor in series with the positive input,
integrator input bias current can be cancelled.
 However, the output still saturates due to other effects such
as input mismatch, etc.
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Speed Limitation
Vout
A0
s  
s
Vin1  Vin 2
1
1
 Due to internal capacitances, the gain of op amps begins to
roll off.
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Bandwidth and Gain Tradeoff
 Having a loop around the op amp (inverting, noninverting,
etc) helps to increase its bandwidth. However, it also
decreases the low frequency gain.
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Slew Rate of Op Amp
 In the linear region, when the input doubles, the output and
the output slope also double. However, when the input is
large, the op amp slews so the output slope is fixed by a
constant current source charging a capacitor.
 This further limits the speed of the op amp.
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Comparison of Settling with and without Slew Rate
 As it can be seen, the settling speed is faster without slew
rate (as determined by the closed-loop time constant).
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Slew Rate Limit on Sinusoidal Signals

dVout
R1 
 V0 1   cos t
dt
 R2 
 As long as the output slope is less than the slew rate, the
op amp can avoid slewing.
 However, as operating frequency and/or amplitude is
increased, the slew rate becomes insufficient and the
output becomes distorted.
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Maximum Op Amp Swing
Vout
Vmax  Vmin
Vmax  Vmin

sin t 
2
2
 FP
SR

Vmax  Vmin
2
 To determine the maximum frequency before op amp slews,
first determine the maximum swing the op amp can have
and divide the slew rate by it.
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Nonzero Output Resistance
A0 
Rout
R1
vout
R1

vin
R2 1  Rout  A  R1
0
R2
R2
 In practical op amps, the output resistance is not zero.
 It can be seen from the closed loop gain that the nonzero
output resistance increases the gain error.
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Design Examples
 Many design problems are presented at the end of
the chapter to study the effects of finite loop gain,
restrictions on peak to peak swing to avoid
slewing, and how to design for a certain gain
error.
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